Earnings Per Share - Schedule of Earnings Per Share from Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ (275.4)	$ 708.7	$ 414.5
Less: total (loss) income from discontinued operations, net of tax	18.7	(2.3)	0.8
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	(294.1)	711.0	413.7
Allocation of (earnings) losses to participating restricted common shares	3.5	(9.3)	(5.3)
Basic and diluted earnings (losses) per share numerators	$ (290.6)	$ 701.7	$ 408.4
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	3,079,000	3,122,200	3,181,600
Average unvested restricted common shares (in shares)	(36,500)	(40,800)	(40,500)
Basic earnings (losses) per share denominator (in shares)	3,042,500	3,081,400	3,141,100
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive options to acquire common shares (in shares)	3,079,000	3,122,200	3,181,600
Average unvested restricted common shares (in shares)	(36,500)	(40,800)	(40,500)
Diluted earnings (loss) per share denominator (in shares)	3,042,500	3,081,400	3,141,100
Diluted earnings per share (in dollars) - continuing operations:
Dividends declared and paid (usd per share)	$ 1.00	$ 1.00	$ 1.00
Undistributed earnings (losses) (usd per share)	(96.53)	226.72	129.02
Basic and diluted earnings (losses) per share (usd per share)	$ (95.53)	$ 227.72	$ 130.02